The present invention relates to an arc tube for a discharge lamp unit of a type which has recently seen increasing use as a bulb for a vehicular headlamp due to its excellent luminous efficiency, color rendering characteristics and long service life compared with an incandescent bulb.
A discharge lamp unit is constructed as shown in FIG. 13. An arc tube 4 is supported by a pair of metal lead supports 2 and 3 which extend from an insulating base 1. The arc tube 4 has a closed glass ball 4a at the middle and pinch-sealed portions 4b on both sides of the closed glass ball 4a formed by pinch-sealing the two open end portions of a quartz glass tube. The closed glass ball 4a defines a discharge section. An electrode assembly 5, composed of a tungsten electrode 5a, a molybdenum foil 5b and a molybdenum lead wire 5c, is sealingly held in each of the pinch-sealed portions 4b in such a manner that the electrodes 5a protrude into the closed glass ball 4a and the lead wires 5c extend outwardly from the pinch-sealed portions 4b. The lead wires 5c are welded to respective ones of the lead supports 2 and 3, which are arc tube supporting members and serve as current supplying lines for the lead wires 5c. Light emitting materials, i.e., mercury and metal iodide together with an inert gas (Xe) are sealingly filled in the discharge section, i.e., in the closed glass ball 4a of the arc tube 4.
In order to light the arc tube stably without extinction of the arc, increase of the firing (arc striking) voltage, or breakage of the arc tube, it is preferable that a voltage of 80 to 90 volts (tube voltage) be applied to the arc tube. To properly operate the arc tube, it is necessary that the output light of the arc tube be suitable in luminous flux, color temperature, and chromaticity. However, no standard ranges have heretofore been established for the quantities of mercury and metal iodide in the closed glass ball and the Xe gas pressure to obtain a suitable luminous flux, color temperature, and chromaticity.